Electrophotographic process using a single phase photoconductive glass imaging layer



United States Patent Int. Cl. G03g U.S. Cl. 96-1 7 4 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic plate is disclosed comprising a single phase glassy layer having at least one photoconductive metal oxide and at least one glass former, with said metal oxide comprising at least 50 mole percent of said layer.

This invention relates in general to electrophotographic imaging systems and, more specifically, to electrophotographic plates having a photoconductive insulating layer comprising a single phase homogeneous glass containing a major proportion of the oxides and of polarizable cations.

It is known that images may be formed and developed on the surface of certain photoconductive insulating materials by electrostatic means. The basic electrophotographic process, as taught by Carlson in U.S. Patent 2,297,691, involves uniformly charging a photoconductive insulating layer and then exposing the layer to light-and-shadow image which dissipates the charge on the portions of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light and shadow image. Alternatively, a latent electrostatic image may be formed on the layer by charging said layer in image configuration. This image is rendered visible by depositing on the imaged layer a finely divided electroscopic developing material. The powder developing material will normally be attracted to those portions of the layer which retain a charge, thereby forming a powder imagecorre'spon'ding to the latent electrostatic image. Where the base sheet is relatively inexpensive, such as paper, the powder image may be fixed directly to the plate, as by heat ornsolvent. fusing. Alternatively, the powder image may be transferred to a sheet of receiving material such as paper and fixed thereon. The above general process is also described in U.S. Patents 2,357,809; 2,891,011 and 3,079,342.

' The photoconductive insulating layer to be useful in electrophotography must be capable of holding an electrostatic charge in the dark and dissipating the charge to a conductive substrate when exposedto light. That various photoconductive insulating materials may be used in making electrophotographic plates is known. Suitable photoconductive insulating materials such as anthracene, sulfur, selenium or mixtures thereof have been disclosed by Carlson in U.S. Patent 2,297,691. These materials generally have sensitivity limited to the blue or near ultraviolet range, and all but selenium have a further limitation of being only slightly light sensitive. For this reason, selenium has been the most commercially acceptable material for use in electrophotographic plates. Vitreous selenium, while desirable in most aspects, suffers from serious limitations in that its spectral response is somewhat limited to the ultraviolet, blue and green regions of the spectrum and the preparation of vitreous selenium plates requires costly and complex procedures such as vacuum evaporation. Also, vitreous selenium layers are.

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only meta-stable in that they are readily recrystallized to inoperative crystalline form at temperatures only slightly in excess of those prevailing in conventional electrophotographic copying machines. Further, selenium plates require the use of a separate conductive substrate layer, preferably with an additional barrier layer deposited thereon before deposition of the selenium photoconductive layer. Because of these economic and commercial considerations, there have been many recent efforts towards developing photoconductive insulating materials other than selenium for use in electrophotographic plates.

It has been proposed that various two-component materials be used in photoconductive insulating layers used in electrophotographic plates. These consist of a photoconductive material in particulate form, dispersed in an insulating binder. Where the particles consist ofa photoconductive material comprising an inorganic crystalline compound containing a metallic ion, satisfactory photographic speed and spectral response for use in electrophotographic plates are obtained. However, these plates, even when dye-sensitized, generally have sensitivities much lower than selenium. These plates are generally considered to be non-resuable since it is necessary to use such high percentages of photoconductive pigment in order to obtain adequate sensitivity that is dilficult to obtain smooth surfaces which lend themselves to efficient toner transfer and subsequent cleaning prior to reuse. An additional drawback in the use of inorganic pigment-binder type plates is that they can be charged only by negative and not by positive corona discharge. This property makes them commercially undesirable since negative corona discharge generates much more ozone and is generally more difiicult to control.

It has been further demonstrated that organic photoconductive dyes in a wide variety of polycyclic compounds may be used together with suitable resin materials to form photoconductive insulating layers useful in bindertype plates. These plates generally lack sensitivity levels necessary for use in conventional electrophotographic copying devices. In addition, these plates lack abrasion resistance and stability of operation, particularly at elevated temperatures.

In another type plate, inherently photoconductive polymers are used, frequently in combination with sensitizing dyes or Lewis acids, to form photoconductive insulating layers. These polymeric organic photoconductive plates generally have the inherent disadvantages of high cost of manufacture, brittleness, poor adhesion to supporting substrates. A number of these photoconductive insulating layers have thermal distortion properties which make them undesirable in an automatic electrophotographic apparatus which often include the powerful lamps and thermal fusing devices which tend to heat the electrophotographic plate.

There has been recently developed a pigment-binder type electrophotographic plate in which the major proportion is a non-photoconductive glass binder. Inorganic photoconductive pigment particles are mixed with glass particles, the glass is fused and the two-phase mixture is coated onto a conductive substrate, forming an electrophotographic plate. Such plates are described in detail by Corrsin in U.S. Patent 3,151,982. These plates have excellent physical characteristics in that they have especially smooth surfaces adapted to easy cleaning and are unusually abrasion resistant. However, these plates have several of the disadvantages of the binder-type plates discussed above. In order to produce a smooth surface plate, no more than about 40 percent by weight of the plate consists of photoconductive particulate material. While it would be desirable to include a larger percentage of photoconductive particles for increased photosensitivity, such plates would have very rough, substantially nonreusable, surfaces. It is also often difficult to obtain a uniform dispersion of the photoconductive particles in the glass binder.

Thus, there is a continuing need for improved photoconductive insulating materials from which stable, highly sensitive, abrasion-resistant and reusable electrophotographic plates can be made.

It is, therefore, an object of this invention to provide an electrophotographic plate devoid of the above-noted disadvantages.

Another object of this invention is to provide an electrophotographic plate having improved homogeneity and uniformity of physical and electrical characteristics.

Another object of this invention is to provide an electrophotographic plate having a smooth, tough, abrasionresistant surface.

Still another object of this invention is to provide an electrophotographic plate having a wide range of useful physical properties.

Yet another object of this invention is to provide elec trophotographic plates suitable for use in both single use and reusable systems.

Still another object of this invention is to provide an electrophotographic imaging process utilizing an electrophotographic plate having high sensitivity and a tough, smooth, abrasion-resistant surface.

It is still another object of this invention to provide a process for preparing an electrophotographic plate having a Wide range of desirable physical properties.

The foregoing objects and others are accomplished in accordance with this invention, fundamentally, by providing an electrophotographic plate comprising a single phase vitreous glass layer, the major component of which is a photoconductive oxide, and a method of electrophotographic imaging using said plate. The plate is homogeneous and non-crystalline and has a smooth, tough surface. Of the photoconductive metal oxides, lead oxide has been found to give especially good results. Plates including lead oxide are especially highly photosensitive and are especially durable. While these plates are sensitive to visible light and are useful in conventional electrophotography, they are also sensitive to X-rays and are especially useful in electroradiography. Systems of electroradiography for which the plates of this invention are suitable include that described by Schaffert in US. Patent 2,666,144, for example. Therefore, lead oxide is the preferred major component for use in single phase glassy plates.

The electrophotographic plate of this invention is distinguished from the inorganic pigment-glass binder plates of the prior art in providing a single phase system which is inherently capable of including a high proportion of photoconductive material without producing a rough surface. The major component of the photoconductive insulating layer consists of the photoconductive metal oxide and the remainder of the layer comprises any glass forming materials which form a homogeneous single phase layer with the metal oxide. It is preferred that the photoconductive insulating layer comprise at least about 50 mole percent photoconductive metal oxide(s) for the optimum combination of desirable physical properties and highest photosensitivity. In general, the higher the proportion of photoconductive metal oxide in the glass, the higher the photosensitivity of the plate. However, if the proportion of somemetal oxide is too high, it may not be possible to prevent the metal oxide from crystallizing. Generally, up to about 85 mole percent metal oxide may be included before undesired crystallization occurs.

Any suitable photoconductive metal oxide may be used in the electrophotographic plates of this invention. Lead oxide has especially desirable electrical and other physical properties and is, therefore, the preferred photoconductive metal oxide. Typical photoconductive oxides include 4 PhD, ZnO, BaO, TiO CdO, Bi O Ga O In O SnO, SnO Sb O TeO Cu O, As O HgO and mixtures thereof.

As the additional glass-forming material to be used with the photoconductive metal oxide in forming electro photographic plates, silicon dioxide has been found to produce excellent single phase glasses with high proportions of the photoconductive metal oxide. Any other suitable glass former may be used where desired. Typical glass forming materials include As O B P 0 Sb O GeO V 0 TeO and mixtures thereof.

The single phase glassy layer may include any other v suitable material, where desired. For example, various physical property modifying materials may be included. Typical materials which may be included to modify the electrical or other physical properties of the plate include A1 0 MgO, Li O, Na O, K 0, CaO, SrO, andmixtures thereof.

The single phase glass photoconductive insulating layer may be deposited on any suitable supporting substrate, or may be cast as a self-supporting sheet. The plate may be over-coated with any suitable material, if desired. The single phase photoconductive insulating layer may be used in the formation of multilayer sandwich configurations adjacent a dielectric layer, similar to those shown by Golovin et al., in the publication entitled A New Elec trophotographic Process, Effected by Means of Combined Electret Layers, Doklady Akad. Nauk SSSR, vol. 129, No. 5, pages 1008-1011, November-December 1959.- Where the single phase glass photoconductive insulating layer is coated on a substrate, a wide variety of materials may be used, for example, metal surfaces such as aluminum, brass, stainless steel, copper, nickel, zinc, etc.; conductively coated glass such as tin or indium coated glass, aluminum coated glass, etc.; under certain conditions, such as at higher temperatures common plate glass has a sufiiciently lower resistivity to act as a ground plane. In general, to act as a ground plane as described herein, a backing material may have a. surprisingly high resistivity, such as 10 -10 ohm/cm. The material must, however, be capable of withstanding the temperatures required for fusing the single phase glass photoconductive insulating layer.

- The following examples further specifically define and describe methods of making the single phase photoconductive glasses of the present invention. Parts and percentages are by weight unless otherwise indicated. The examples below should be considered to illustrate various preferred embodiments of the photoconductive glass preparation processes of this invention.

EXAMPLE I About 25 mole percent powdered SiO is mixed under acetone with about 70 mole percent powdered PhD and 5 .mole percent A1 0 The mixture is placed in a platinum crucible, and melted at a temperature of about 900 C. A clear, single phase glass is formed in the crucible. The melt is poured onto a stainless steel plate, preheated to about 200 C. The glass is then annealed from about 400 C. to room temperature in about 3 hours. The glass layer produced is then examined with a polarizing microscope and by X-ray diffraction, both of which establish the absence of crystalline phases. The resulting glass has a tough, abrasion-resistant surface and a pale yellow color.

EXAMPLE 11 About 23 mole percent powdered SiO about 60 mole percent powdered PhD and about 17 mole percent Bi O are mixed under acetone. The mixture is placed in a platinum crucible and heated to about 900 C. As the powders melt, a clear single phase glass is formed. The melt is poured onto a stainless steel plate preheated to about 200 C. The glass is then annealed from about 400 C. to room temperature in about 3 hours. A tough abrasion resistant glass results with a pale yellow color.

5 EXAMPLE III EXAMPLE IV About 20 mole percent powdered B is mixed under acetone with about 70 mole percent powdered CdO and about 10 mole percent powdered A1 0 The mixture is placed in a platinum crucible and melted at a temperature of about 1300 C. The melt is poured onto a stainless steel plate preheated to about 200 C. The glass is then annealed from about 500 C. to room temperature, in about 4 hours. Examination with a polarizing microscope and X-ray diffraction establish the absence of crystalline phases. A smooth, hard surface glass results having a dark color.

EXAMPLE V About 10 mole percent powdered SiO is mixed with about 12 mole percent powdered B 0 about 0 mole percent powdered PbO and about 28 mole percent ZnO under acetone. The mixture is placed in a platinum cruicible and melted at a temperature of about 950 C. The melt is poured onto a stainless steel plate preheated to about 200 C. The 'glass is annealed from about 450 C. to room temperature in about 3 hours. Examination confirms the presence of a single phase glassy layer. This plate has a pale yellow color.

EXAMPLE VI The following ingredients are mixed in powdered form under acetone: about mole percent CdO, about 5 mole percent ZnO, about 35 mole percent PbO, about 5 mole percent A1 0 about 5 mole percent Bi O about 5 mole percent B 0 about 25 mole percent Si0 and about 10 mole percent GeO The mixture is placed in a platinum crucible and melted at a temperature of about 950 C. The melt is poured onto a stainless steel plate preheated to about 300 C. The glass is annealed from about 45 0 C. to room temperature in about 3 hours. A single phase glassy layer results.

The single phase glalss photobonduc'tive insulalting layers prepared as in the above examples are useful in electrophotographic plates in electrophotographic imaging processes. These materials are also especially useful in electroradiographic imaging processes.

The following examples further specifically define the present invention with respect to the single phase glass electrophotographic plates and imaging processes using them. Parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the electrophotographic plates and electrophotographic imaging processes of the present invention.

EXAMPLE VII A sheet of glass prepared as in Example I having a thickness of about 330 microns is bonded to an aluminum substrate with Silverprint, a mixture of silver powder in an adhesive carrier, available from General Cement Electronics Co. The plate is charged to a potential of about 2,000 volts in the dark by corona discharge means such as is described in U.S. Patent 2,777,957. A conventional black and white transparency is placed about 1 millimeter from the plate surface. This composite is illuminated with a 100 watt Burton lamp available from the Burton Manufacturing Company. The lamp is held at a distance of about 1 inch from the plate. The plate is exposed to the image for about 30 seconds. The image is developed by cascading an electroscopic marking material over its surface by the method described by Walkup in U.S. Patent 2,618,551. An image is formed on the plate surface conforming to the transparency image. The powdered image is transferred to a sheet of ordinary bond paper by the electrostatic transfer method described by Schatfert in US.. Patent 2,576,047. The paper sheet is then heated to the melting point of the electroscopic marking particles and cooled; forming a permanent image of good quality conforming to the original. The plate may then be reimaged by the above process.

EXAMPLE VIII A 700 micron sheet of glass prepared as in Example II above is bonded to an aluminum substrate with Silverprint. The plate is then charged to a potential of about 2,500 volts by corona discharge means. The charged plate is exposed to an image through a conventional black and white transparency. The illumination is provided by a Watt Burton lamp held about 1 inch from the plate for about 60 seconds. The latent electrostatic image thus formed on the plate surface is cascade developed. The powder image formed on the plate surface is transferred to a paper sheet and fixed thereon. A good image conforming to the original results. The plate may then be reused, as by the above process.

EXAMPLE IX A 1,100 micron glass sheet prepared as in Example III above is bonded to an aluminum substrate with Silverprint. The plate is charged to a potential of about 2,500 volts by corona discharge. The charged plate is exposed to a light image through a conventional black and White transparency. The plate is illuminated by a 100 watt Burton lamp held about 1 inch from the plate surface for about 60 seconds. The latent electrostatic image on the plate is then cascade developed. The powder image is. then fixed directly to the plate by heating the plate above the fusing temperautre of the electroscopic marking particles. An excellent image corresponding to the original is produced.

EXAMPLE X A 900 micron glass sheet prepared as in Example 1V above is bonded to an aluminum substrate with Silverprint. The plate is charged to a potential of about 2,500 volts by corona discharge. The charged plate is exposed to a light image through a conventional black-and-white transparency. The plate is illuminated by a 100 watt Burton lamp held about 1 inch from the plate surface for about 60 seconds. The latent electrostatic image on the plate is developed by cascade. The powder image is fixed directly to the plate by heating the plate above the fusing temperature of the electroscopic marking particles. An image of good resolution and density corresponding to the original results.

EXAMPLE XI A 700 micron glass sheet prepared as in Example V above is bonded to an aluminum substrate with Silverprint. The plate is charged to a negative potential of about 2,500 volts by corona discharge. The charged plate is exposed and developed as in Example VII above. An image of good quality corresponding to the original results.

EXAM-PIE XII A 700 micron glass sheet prepared as in Example VI above is bonded to an aluminum substrate with Silverprint. The plate is charged to a negative potential of about 2,000 volts by corona discharge. The charged plate is exposed and developed as in Example IX above. An image of excellent quality corresponding to the original results.

Although specific components and proportions have been described in the above examples of methods of preparing photoconductive single phase glasses and of forming electrophotographic images on single phase glass plates, other suitable materials, as listed above, may be used with similar results. In addition, other materials may be added to the glass compositions to synergize, enhance, or otherwise modify their properties. For example, spectral sensitizing agents may be added to the glass compositions to modify the spectral response of the plates.

Other modifications and ramifications of the present invention will occur to those skilled in the art upon a reading of the present disclosure. These are intended to be included within the scope of this invention.

What is claimed is:

1. The process for forming a latent electrostatic charge pattern on a surface which comprises:

(a) providing an electrophotographic plate comprising a layer consisting essentially of a single phase photoconductive glass comprising at least one photoconductive metal oxide selected from the group consisting of PbO, AS203, ZnO, BaO, TiO CdO, Bi O 63203, 111203, SnO, SHO'g, Sb203, T302, C1120, and HgO, and mixtures thereof in an amount from about 50 to about 85 mole percent; and at least one glass former selected from the group consisting of SiO G60 B203, V205, P205, T60 AS203, Sb O and mixtures thereof;

(b) electrostatically charging the surface of said plate;

and

(c) exposing said surface to a patent of activating electromagnetic radiation to form a latent electrostatic image.

2. The process of claim 1 wherein the latent image is developed to form a visible image.

3. An electrophotographic process comprising:

(a) providing an electrophotographic plate comprising a layer consisting essentially of a single phase photoconducti-ve glass comprising at least one photoconductive metal oxide selected from the group consisting of PbO, AS203, ZnO, BaO, TiO' CdO, Bigog,

s 63.203, 111203, 8110, SHOZ, $13 TeOg, C1420, and mixtures thereof in an amount from about "50 to about 85 mole percent; and at least one glass former selected from the group consisting of Slo 5 mixtures thereof; l Y

(b) forming an electrostatic latent. image on the surface of said plate; and p I I (c) developing said electrostatic latent image with electroscopic marking particles. 7

4. An electrophotographic process of claim 3 wherein the plate is passed at least twice througha cycle comprising charging, image exposure, and image developing.

References Cited UNITED STATES PATENTS 2,210,489 8/1940 Lemmens r a1. 176-16 2,220,765 11/1940 .Huoseet a1 106 36.1 2,297,453 9/1942 Berger 6t al. 106-54 2,435,995 2/1948 Armstead 106-52 2,461,878 2/1949 Christensen et al. 171-4 327 2,490,331 12/1949 Armistead 10653 2,937,100 5/1960 Oldfield et al. 106 53 3,149,984 9/1964 Faulstich 106-53 3,151,982 10/1964 Corrsin 9-6-4 3,294,561 12/1966 Duncan 1O654.

OTHER REFERENCES Phase Diagrams for Ceramists, by Levin, et al. Published by Am. Ceramic Soc. Inc., pp. 116, 117,120

GEORGE F. LESMES, Primary Examiner I. C. COOPER III, Assistant Examiner US. 01. x11.

G602, B203, V205, P205, T602, AS203, andi 

